A hydrolock ( a shorthand notation for hydrostatic lock or hydraulic lock ) is an abnormal position of any device designed to compress a gas by mechanically stopping it; Most commonly reciprocating internal combustion engines, this article refers to the case unless otherwise noted. Hydrolock occurs when an amount of liquid that exceeds the volume of the cylinder at the cylinder’s minimum (end of the piston stroke) enters the cylinder. Since fluids are almost incompressible the piston cannot complete its travel; Either the engine must stop spinning or there must be a mechanical failure.

Symptoms and disadvantages

If an engine hydrolocks while in motion, there is a potential for mechanical failure. Common damage modes include a bent or broken connecting rod, a fractured crank, a fractured head, a fractured block, crankcase damage, damaged bearings, or any combination of these. Forces absorbed by other interconnected components can cause additional damage. Physical damage to metal parts may manifest as a “crashing” or “screaming” sound and usually requires replacement of the engine or substantial rebuilding of its major components.

If an internal combustion engine hydrolocks at idle or low power conditions, the engine may stop suddenly without causing any immediate damage. In this case the engine can often be purged by removing the spark plug or injector and turning the engine to drain the liquid out of the combustion chambers after which a restart can be attempted. Depending on how the liquid was brought into the engine, it may be resumed and drained with normal combustion heat, or it may require more work, such as draining out contaminated operating fluid and Replace damaged gasket.

If one cylinder fills with liquid when the engine is turned off, the engine will refuse to start when the engine is attempted to be started. Since the torque of the starter mechanism is usually much less than the operating torque of the engine, this will usually not damage the engine but may burn the starter. The engine can be taken out and restarted as above. If corrosive substances such as water have been rusting in the engine for a long time, more extensive repairs will be required.

The amount of water sufficient to cause hydrolock disturbs the air/fuel mixture in a gasoline engine. If water is introduced slowly enough, this effect can reduce power and speed in an engine to such an extent that it does not cause catastrophic damage to the engine when a hydrolock actually occurs.

Cause and special cases

Motor vehicle

Hydrolock most commonly occurs in automobiles when driving through a floodplain, either where the water is above the level of the air intake or the vehicle speed is excessive, producing a long bow wave . A vehicle with a cold air intake installed low on the vehicle will be particularly vulnerable to hydrolocking when driven through standing water or heavy rainfall. Engine coolant entering the cylinders through various means (such as a blown head gasket ) is another common cause. Excessive fuel ingress ( flooding ) in one or more cylinders in liquid form due to abnormal operating conditions can also cause hydrolock.


Small boats with outboard engines and PWC swallow water simply because they run in and around it . During a rollover, or when a wave washes over the craft, its engine can hydrolock, although serious damage is rare due to the specialized air intakes and low rotational inertia of small marine engines. Inboard marine engines have a distinct vulnerability because they often mix their own cooling water with the exhaust gases in the header to cool the engine. A rusted exhaust header or a long period of starting the starter can cause water to build up in the exhaust line to the point that it flows back through the exhaust manifold and fills the cylinders. [1]The intercooler on a turbocharged engine is normally cooled by sea water, so if it rusts, the water will be swallowed by the engine.

Diesel engine

Diesel engines are more sensitive to hydrolock than gasoline engines. Due to their high compression ratio, diesel engines have a very small final combustion chamber volume, requiring very little liquid to hydrolock. Diesel engines also have higher torque , rotational inertia and stronger starter motors than gasoline engines . The result is that a diesel engine is more prone to catastrophic damage.

Radial and inverted engines

Hydrolock is common on radial and inverted engines (cylinders pointing down) when the engine is seated for long periods . Engine oil seeps into the cylinder under gravity through various mediums (via rings, valve guides, etc.) and can fill a cylinder with enough oil to hydrolock. The leakage effect can be seen from the bluish-white smoke usually seen when the radial engine is running. To prevent engine damage, it is universal practice for the ground crew or pilot to check the hydrolock during a pre-flight inspection of the aircraft, usually by slowly cranking the propeller for several turns, either by hand or by the starter. by using motor. Make sure the crankshaft cycles normally through all cylinders.

Steam engines

The steam engine may have a hydraulic lock as the steam condenses back into water . In most steam engine designs there should be a small moment at the end of the return stroke of the piston when all valves are closed and it is compressing any remaining steam. Water may be introduced from the boiler or into a cold engine, the steam will condense to water on the cold walls of the cylinder and potentially hydrolock an engine. It is as harmful as it is for internal combustion engines and can be very dangerous in the case of steam engines as a broken connecting rod can puncture the firebox or boiler and cause a steam explosion. Steam engines (with the exception of smaller models and toy machines) always have cylinder drain cocks that are opened to allow excess water and steam to escape during warm-up.